It is known that a diesel engine of a motor vehicle conventionally includes an engine block defining at least one cylinder having a piston, and a cylinder head that closes the cylinder and cooperates with the piston to define a combustion chamber. A fuel and air mixture is disposed in the combustion chamber and ignited, resulting in hot expanding exhaust gasses that causes reciprocal movements of the piston, thereby rotating a crankshaft.
After the expansion, the exhaust gases exit the combustion chamber and are directed into an exhaust system which conventionally include an exhaust pipe having one or more exhaust after treatment devices. The after treatment devices may be any device configured to change the composition of the exhaust gases, in order to reduce the polluting emissions of the engine. Among these after treatment devices, the exhaust system may include a selective catalytic reduction (SCR) system.
An SCR system usually includes a catalytic device in which the nitrogen oxides (NOx) contained in the exhaust gases are reduced into diatonic nitrogen (N2) and water (H2O), with the aid of a gaseous reducing agent that is absorbed inside catalyst. This reducing agent is obtained by injecting a Diesel Exhaust Fluid (DEF) into the exhaust pipe, upstream of the catalytic device. The DEF is generally an aqueous solution of urea (CH4N2O) which mixes with the hot exhaust gases and is converted thereby into ammonia (NH3).
The DEF is injected into the exhaust pipe by one DEF injector, which is in fluid communication with a DEF pump that increases the pressure of the DEF received from a DEF tank. The DEF injector may be a conventional solenoid injector that includes an external casing having a nozzle, a valve member shaped as a needle located in the external casing, a spring biasing the needle to close the nozzle, and a solenoid (e.g. electric coil) that can be energized to generate a magnetic field that moves the needle to open the nozzle and let the DEF into the exhaust pipe. The opening of the DEF injector is usually commanded by an Electronic Control Unit (ECU), following known strategies that determine a target or requested DEF quantity to be injected and energize the solenoid of the DEF injector accordingly.
A drawback of the SCR system is that the DEF injector, being located in direct contact with the exhaust gases flowing in the exhaust pipe, is sometimes heated up to very high temperatures, which may affect its operation and also cause irreversible damage to its nozzle and needle.
However, the DEF injectors currently used in the SCR systems are not provided with a temperature sensor, so that the ECU cannot implement any protection strategy to prevent the above mentioned drawback.